1. Prior art
A conventional type die cushion apparatus is shown in FIG. 5. A piston 102 is incorporated in an air cylinder 101 so that it can freely go up and down, and a wear plate 104 is fixed on an upper portion of a piston rod 103. The wear plate 104 receives the lower surface of a cushion pin (not shown), which is inserted into female mold, and the blank holder pressure is transmitted to the cushion pin by the air cylinder 101 and the piston 102. An upper chamber 101U and a lower chamber 101L are formed by air cylinder 101 and piston 102, and the upper chamber 101U communicates with atmospheric air through an opening 105. The lower chamber 101L communicates with an air tank 106 through a connection pipe 107. The air tank 106 is connected to an air supply circuit 108, and compressed air of a predetermined pressure is supplied.
When press operation (e.g. drawing) is performed by this die cushion equipment, the compressed air corresponding to the blank holder pressure required is supplied to the air tank 106.
The compressed air with lower chamber pressure PL satisfying the condition: EQU Blank holder force F =(Effective pressed area Al of piston 102) .times.(Pressure in lower chamber PL)
is supplied. As the slide of the press goes down, the piston 102 goes down through cushion pin, wear plate, etc., a blank holder force is generated. When the piston 102 comes to the lower limit (at the lowest position), the drawing process is completed. Then, the slide of the press goes up, and wear plate 104, piston 102, etc. also go up. (Problems to be solved by the invention)
In the above die cushion equipment of conventional type, it is discussed here how the blank holder force is changed when the piston 102 goes down. FIG. 4 shows the operation of the apparatus according to this invention and that of the conventional type equipment. The stroke of the piston 102 is given on the abscissa, and the blank holder force is shown on the ordinate. Namely, the abscissa shows the position when the piston 102 moves from the upper limit w (the highest position) to the lower limit LL, and the ordinate give the blank holder force generated by the piston 102 (such as F, Fa, Fae, etc.).
When it is supposed that the necessary blank holder force is Fa, the blank holder force is Fae when the piston 102 reaches the lower limit. The blank holder force at the lower limit is increased by Fae-Fa compared with the value at the upper limit because the capacity of the lower chamber 101L is decreased as the piston 102 goes down.
It is not desirable that the blank holder increases in the drawing process. Therefore, an air tank with large capacity air tank 106 is furnished in the past. The capacity should be as large as possible. It is about 5-8 times as high as that of the air cylinder 101 in a normal case because of limits on the space available for installation. Accordingly, there have been the following problems with the die cushion equipment of the conventional type:
(1) A large capacity air tank 106 is required, and it is not very easy to keep the space of installation. PA0 (2) Even when the large capacity air tank 106 is furnished, it is impossible to reduce the increase of the blank holder force to zero. PA0 (3) Air pressure in the air tank 106 must be adjusted when a die is replaced. This requires a long time because of the large capacity. When it is adjusted by decreasing the air pressure, compressed air must be discharged and this is not very economical. (4) When the blank holder force is to be increased or decreased as the piston 102 goes down in the drawing process, it cannot be freely increased or decreased.
Especially, a serious problem arises in a case of a transfer press because a large number of die cushion apparatus are to be installed.